The present disclosure relates to semiconductor devices, and in particular, to highly-integrated three-dimensional semiconductor memory devices. Higher integration of semiconductor devices may be used to address consumer demands for superior performance and inexpensive prices. In the case of semiconductor devices, since their integration may be an important factor in determining product prices, increased integration is especially desirable. In the case of two-dimensional or planar semiconductor devices, since their integration is mainly determined by the area occupied by a unit memory cell, integration can be greatly influenced by the level of a fine pattern forming technology. However, the expensive process equipment used to increase pattern fineness sets a practical limitation on increasing integration for two-dimensional or planar semiconductor devices. To overcome such a limitation, three-dimensional semiconductor memory devices including three-dimensionally arranged memory cells have recently been proposed.